Aseptic food products are processed by heating the food in a sterile environment at a temperature and time sufficient to kill a reference microorganism population, and thereafter maintaining the sterility during the filling and packaging operations. The unit of measurement, Fo, designates heat treatment of one minute at 121.1 degrees Centigrade referenced to z=10 degrees Centigrade. Another unit of measurement, D, indicates in logrithmic scale, the time of heating, at temperature T, needed to destroy 90% of a designated contaminant population. An Fo equal to 2.45 minutes is a generally accepted minimum design criterion for a 12 D destruction for Clostridium botulinum. However, recognizing that spoilage generally occurs due to the survival of heat resistant spore-forming bacteria more resistant than Clostridium botulinum, an Fo equal to five minutes has been generally accepted for low acid, aseptically processed and packaged foods. There is a logrithmic relationship between sterilizing time and temperature, therefore the higher the processing temperature, the less processing time is necessary to produce a sterilized product. In addition, there is a product quality advantage in using a higher processing temperature because microorganisms are more temperature sensitive than are many desirable components of food. This advantage is explained by comparing the slope (z) of the thermal death time curve (TDT) with similar curves for desirable components of food such as thiamin, chlorophyll, and ascorbic acid. Thermal death time is the time to achieve sterility of a suspension containing a known number of cells or spores at any predetermined temperature.
Conventional aseptic food processing systems are typically batch based and typically sterilize food in liquid in a closed system. These systems typically heat food particulates and prepackaged products to the requisite times and temperatures by using pumps, elevated pressure enclosures, and heat exchangers. The products are generally placed in a surface heat exchanger or in a sealed chamber and heated in a gaseous environment and pressure cooked. The scrape surface heat exchanger tends to mash, shear, or tear the product which may yield an unacceptable product. In addition, the conventional batch heating systems have processing times which include the batch heating and cooling process times.
A continuous sterilization process which is open to atmospheric pressure reduces both the process time and amount of energy currently needed to run the batch type closed systems. A liquid column increases the boiling point of the liquid in the sterilization zone and allows the product to be both sterilized and cooked while being continuously transported through the heated liquid to the aseptic zone. The liquid in the sterilization zone acts as a barrier to the aseptic zone. In addition, the integrity and texture of the product is preserved by the reduced processing time. The increased temperature and reduced process time will improve the shelf-life, safety, and nutritional aspects of the sterilized product.